The auditory cortex is a complex layered structure whose circuitry is composed of a variety of cell types and their interconnections. Auditory information form thalamus terminates in layers III and IV and extracellular recordings from cells there reveal a diverse set of responses to auditory stimuli. The long-term objective of this project are to understand how cells in these layers integrate and process the most basic features of this ascending information and where they subsequently send it. The basic approach involves using single cell intracellular recording methods to record from and characterize the sub- and suprathreshold responses of cells, either in vivo or in the brain slice, followed by labeling of the cells for subsequent anatomical study. In vivo we will use simple auditory stimuli to test 1) whether the cells are excited by stimulation of either ear ("EE") or excited by the contralateral ear and inhibited by simultaneous stimulation of the ipsilateral ear ("EI") 2) the monotonic (progressive increase in spike output with increasing SPL from threshold to saturation) or non- monotonic (maximum spike output at intermediate SPL levels that drops off at higher and lower intensities) nature of the cells and 3) whether cells are sensitive to either the duration of a single stimulus or the duration of the interval between two stimuli. Such an approach determines 1) if the set of cells that display a unique response to a particular auditory signal is a specific anatomical class, 2) what synaptic inputs are shaping these responses? Are both excitatory and inhibitory events involved or is the cell simply mimicking a suprathreshold ascending input? 3) whether the cell's intrinsic membrane features are important in letting the cell respond optimally to it's chosen stimuli and 4) what the cell does with its information, i.e. what are the projection patterns of the axon? In the brain slice we will record from the same populations of cells and, because of the nature of slice recording, be able to more closely study the physiology and pharmacology of the synaptic inputs and the biophysical properties of the cells. These results will begin to lay a structural /functional framework on which to build an understanding for higher auditory cortical function.